1. Field of the Invention
This invention relates to a method of producing finely divided polymer particles, in particular, to such a method which comprises first dispersing as an oil phase a polymer solution containing one or more kinds of polymers into an aqueous dispersing medium with suitable control of the viscosity as well as the surface tension of each phase and then removing the solvent included in the oil phase droplets, thus obtaining spherical, finely divided polymer particles having high quality and shape and particle size uniformity.
2. Description of the Prior Art
Various methods of producing finely divided polymer particles are known in the art. In one method based on suspension polymerization, for example, one can obtain particles of from about 1,000 microns to 10 microns in diameter by selection of a suitable dispersion stabilizer and appropriate control of the concentration of polymerization initiator, the polymerization temperature, the weight ratio of monomer to water, etc. However, it is quite difficult to produce particles finer than 10 microns in diameter having a narrow size distribution. In the 10 microns in diameter having a narrow size distribution. In the above cited suspension polymerization method, there are two methods for dispersion stabilization for the oil phase suspended in the aqueous medium; one involves using a water soluble polymer dispersion stabilizer, and the other involves using a water insoluble inorganic powder dispersion stabilizer. In the former method, however, the preparation of particles of a narrow size distribution with a mean diameter smaller than 10 microns has been extremely difficult and no description has as yet been found in any publication relating to such technology. In the latter method employing a water insoluble inorganic powder dispersion stabilizer, a similar technological difficulty exists and only a few practical methods under rather limited conditions are known; for example, Japanese Patent Publication 51,830/1972 discloses a method in which an inorganic dispersion stabilizer such as magnesium carbonate, zinc oxide, magnesium phosphate, etc., and a flow aid (often term a fluidizing agent in the art) such as ethyl cellulose are first blended with monomers, and then the monomers are suspension polymerized in an aqueous medium to give polymer particles of less than 10 microns. However, this method involves a large amount of dispersion stabilizer and complicated procedures such as blending and removal of the stabilizer with acid. Further, since this method can be used under only a very limited range of polymerization conditions, which, in turn, depend on the reactivities of the monomers involved, it is only applicable to limited monomer combinations.
Emulsion polymerization provides polymer particles having a uniform particle diameter of around 2 to 0.01 micron by suitable control of the amount of emulsifier (surface active agent), polymerization temperature, and stirring conditions, but can hardly form particles greater than 2 microns. Mechanical pulverization followed by classification can produce polymer particles where the size distribution is broad and the shape is not spherical but quite irregular. One can also produce spherical polymer particles by dissolving a polymer in a water immiscible organic solvent and jetting it from a fine nozzle into an aqueous medium under high pressure. However, the particle size is far from uniform and apparatus of high volume is required. In summary, there is no generally useful economical method of producing polymer particles having particle size ranging from 1 to 10 microns.
On the other hand, polymer particles of about such a size range have wide industrial applications including additives for photographic sensitive materials (added to the outermost layer or back coating layer thereof in order to prevent blocking), ion exchange resins, powders for electrostatic coating, colorants (which can be prepared from such polymer particles with the incorporation of pigments), binders for lubricants, cosmetics, fillers for liquid chromatography columns, standard particles for particle size analysis, etc.